Ring cavities, as traveling wave cavities, are inherently superior compared with standing wave cavities, so that the ring cavities are widely used as passive linear devices (such as filters, dispersion compensators and sensors), active linear devices (such as lasers, modulators and detectors), or nonlinear devices (cavity quantum electrodynamics). Especially passive ring cavities, which have no inter-coupling resulted from gain competition so that high-quality traveling waves may be realized and high selectivity of wavelength and slow light behaviors may be thus obtained, are widely used in many fields (including lasers or optic gyroscopes or the like). Therefore, the passive ring cavities may be made of many kinds of material such as glass, polymer, SOI, SiN, SiO2, Si, LiNbO3 and III-V. However, without exception, loss in the ring cavities becomes a factor for restricting the implementation of high-performance passive ring cavities.
In order to reduce the loss, ring cavities consist of a large-diameter circular ring based on low-refractive index transparent material or of a rectangular ring based on transparent material. Employing one of those measures will somewhat reduce the loss and improve the performance of the device. However, on one hand, such change will bring about other relevant problems, and in this case, trade-off is the final choice; and on the other hand, scattering loss, which can not be eliminated in any way, becomes the final factor for restricting the performance of such devices.
Therefore, how to design a ring cavity which can reduce or even eliminate the loss in the cavity and further improve its quality factor so that the ring cavity can fully play its functions, becomes a new research and development direction.